Spectroscopic temperature measurement

ABSTRACT

A method and apparatus for determining the temperature of gaseous materials are provided. Light produced by scattering in a gaseous material and having spectral components periodic in frequency is collected, collimated and transmitted by a light conditioning means to an interferometric means. The interferometric means selectively separates periodic spectra from the light and transmits the spectra in the form of a detectable signal containing first and second branches of the spectra. Means are provided for measuring the intensities of the branches and detecting and recovering the intensity ratio thereof, which is correlated with the temperature of the gaseous material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 584,085, filed June 5, 1975,now U.S. Pat. No. 4,018,529, which in turn is a continuation-in-part ofapplication Ser. No. 478,405, filed June 11, 1974, now U.S. Pat. No.3,909,132.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for measuring thetemperature of gaseous materials through the selective transmission oftheir periodic spectra.

DESCRIPTION OF THE PRIOR ART

In the apparatus conventionally used for spectroscopic measurement ofgas temperatures, light produced by scattering in the gas is collectedand transmitted to a spectrometer whose pass band is scanned to transmitsequentially the rotational Raman spectra of the gas. The intensity ofeach spectral line is recorded as a function of frequency and used tocalculate the temperature of the gas. It has also been proposed tospectroscopically measure gas temperature by transmitting such scatteredlight to a beam splitter associated with a pair of interference filtersadapted to transmit single spectral lines or bands thereof frompreselected portions of the rotational Raman spectrum of the gas. Anintensity ratio derived from the output signals of the interferencefilters is used to calculate the temperature of the gas.

One of the major problems with such apparatus is the difficulty ofaccurately measuring the temperature of gases present at remotelocations. The output signal from the spectrometer represents arelatively low intensity signal that is frequently obscurred by spectralinterference between rotational Raman spectra of the gas being measuredand spectra of coexistent gases. Use of a beam splitter reduces theamount of light transmitted to and hence the intensity of the outputsignal from each interference filter associated therewith. Moreover, inorder to minimize the aforesaid spectral interference, the interferencefilters are adjusted to transmit relatively low intensity signalsderived from limited portions of the spectrum. For the above reasons,rotational Raman scattering is often too insensitive for measurement oftemperatures of gases present at remote locations.

SUMMARY OF THE INVENTION

The present invention provides apparatus having increased sensitivityfor spectroscopically measuring the temperature of gaseous materials.Such apparatus has light conditioning means for collecting, collimatingand transmitting light produced by scattering in gaseous material andhaving spectral components periodic in frequency. An interferometricmeans adapted to receive such light selectively separates periodicspectra therefrom and transmits the spectra in the form of a detectablesignal correlated with the temperature of the gaseous material. Suchinterferometric means has interference-producing means for providing aplurality of transmission windows regularly spaced in frequency. Thefrequency spacing between adjacent windows, or spectral range, of theinterferometric means is adjusted to depart from an odd integralsubmultiple, n, of the frequency difference between adjacent spectralcomponents of the periodic spectrum of a molecular species of thegaseous material, said odd integral submultiple being at least three, soas to produce a split-fringe containing first and second branches of thecomponents. Such interferometric means also has scanning means forcausing the transmission peaks for adjacent nth orders to substantiallycoincide with the spectral lines of either branch of the components.Each branch of the split-fringe is derived from a plurality of periodicspectral lines and has an integrated intensity substantially equal totheir sum. The intensity of each of the branches of the split-fringe ismeasured by a signal conditioning means, and the intensity ratio of thebranches is indicated and recorded by detecting means, the intensityratio correlating with the temprature of the gaseous material.

Further, the invention provides a method for determining the temperatureof a gaseous material by analyzing light having spectral componentsperiodic in frequency, comprising the steps of collecting, collimatingand transmitting the light in the form of a ray path;interferometrically separating periodic spectra from the light bydirecting the light through a plurality of transmission windowsregularly spaced in frequency, the frequency spacing between adjacentwindows being adjusted to depart from an odd integral submultiple, n, ofthe frequency difference between adjacent spectral components of theperiodic spectrum of the gaseous material, or a constituent thereof ifthe gaseous material comprises a mixture of gases, said odd integralsubmultiple being at least three, so as to produce a split-fringecontaining first and second branches of the components, and scanning theray path to cause the transmission peaks for adjacent nth orders tosubstantially coincide with the spectral lines of either branch of thecomponents; transmitting a detectable signal composed of thesplit-fringe, each branch of the split-fringe being derived from aplurality of spectral lines and having an inegrated intensitysubstantially equal to their sum; measuring the intensity of each of thebranches; and detecting and indicating the intensity ratio of thebranches, the intensity ratio being correlated with the temperature ofthe gaseous material.

Although the light which is subjected to analysis can be received froman external source, it is usually produced by the apparatus. Thus, theapparatus preferably has light source means for generating monochromaticlight. A projecting means associated with the light source means directsthe monochromatic light through the gaseous material to producescattered light having spectral components periodic in frequency. Lightconditioning means are provided for collecting, collimating andtransmitting the scattered light to an interferometric means of the typedescribed.

Several known interferometric means may be adapted for use with theabove apparatus. Preferably, the interferometric means is a Fabry-Perotinterferometer (FPI) having a mirror separation, d, adjusted to transmitall rotational lines of a molecular species, or constituent, of thegaseous material in the form of a detectable signal correlated with thetemperature thereof. This condition obtains when ##EQU1## where d is themirror separation of the FPI, n is an odd integer, μ is the index ofrefraction of the medium between the mirrors, and B is the molecularrotational constant of the species. For a given molecular species, therotational constant B and mirror separations d for transmitting all therotational Raman lines of the species are unique quantities. Theintensity distribution of the transmitted spectra varies directly withthe temperature of the species. Hence, the temperature of the speciesproducing a particular rotation Raman spectrum is determined byadjusting the mirror sparation of the FPI to transmit all rotationalRaman spectra of the species in the form of a split-fringe containing afirst branch (composed of Stokes rotational lines) and a second branch(composed of anti-Stokes rotational lines), measuring the peak intensityof each branch and determining the intensity ratio of the branches.Advantageously, the throughput of the FPI is considerably greater thanthat for a spectrometer or for a beam splitter associated with a pair ofinterference filters. Moreover, the detected signal has a pair ofbranches each of which is derived from a plurality of spectral lines andhas an integrated intensity substantially equal to their sum. Spectralinterference is minimized. The sensitivity of the apparatus is increasedand highly sensitive forms and combinations of detectors, light sources,filters and control systems are unnecessary. As a result, the method andapparatus of this invention permits gas temperatures to be measured moreaccurately and at less expense than systems wherein the spectra aretransmitted sequentially or from limited portions of the spectrum.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings in which:

FIG. 1 is a block diagram showing apparatus for determining thetemperature of gaseous material by analyzing light having spectralcomponents periodic in frequency;

FIG. 2 is a schematic diagram of the apparatus of FIG. 1 including,additionally, means for producing light carrying such spectra;

FIG. 3 is a side view, partially cut away, showing means for modulatingthe interferometric means of FIGS. 1 and 2;

FIG. 4 is a graph showing schematically the peak intensities ofpreselected spectral components and their relative positions within agiven fringe;

FIG. 5 is a graph showing schematically a split-fringe profile for thespectral components of FIG. 4;

FIG. 6 is a graph showing a computed split-fringe profile for nitrogengas;

FIG. 7 is a graph showing calculated intensity ratios for differentfringe numbers of the split-fringe for nitrogen gas at temperatures of200° K, 300° K and 400° K;

FIG. 8 is a graph showing calculated intensity ratios of split-fringesfor nitrogen gas within the range of 100° K-600° K and values of thefree spectral range in the vicinity of 4B/5;

FIG. 9 is a graph showing calculated intensity ratios of thesplit-fringes for nitrogen gas within the temperature range of 100°K-600° K and at values of the free spectral range in the vicinity of 4B.

FIG. 10 is a graph showing computed variations of the frequencydifferece between peak portions of a split-fringe for nitrogen gaswithin the temperature range of 100°-1000° K and at values of the freespectral range in the vicinity of 4B; and

FIG. 11 is a graph showing computed variations of the frequencydifference between peak portions of a split-fringe for nitrogen gaswithin the temperature range of 100°-600° K and at values of the freespectral range in the vicinity of 4B/5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Light scattered from gaseous material and having spectral componentsperiodic in frequency can be produced in each of the visible, infraredand ultraviolet frequency regions at intensities sufficient to measurethe temperature of the gaseous material. As a consequence, the inventionwill function with light having a relatively wide range of frequencies.For illustrative purposes, the invention is described in connection withapparatus for measuring temperature of gaseous material by analyzingscattered light from the visible frequency region. When applied in thismanner, the invention is particularly suited to measure the temperatureof a gas mixture such as air. It will be readily appreciated that theinvention can be practiced using light from any of the foregoingfrequency regions, and that it can be employed for similar and yetdiversified uses, such as remote detection of clear air turbulence,weather forecasting, gas stream analysis, industrial process controlsystems and the like.

Referring to FIG. 1 of the drawings, there is shown preferred apparatusfor measuring the temperature of gaseous material. The apparatus, showngenerally at 10, has light conditioning means 12 for collecting,collimating and transmitting light 14 having spectral componentsperiodic in frequency. An interferometric means 16 receives the light14, selectively separates preselected spectra therefrom and transmitsthe spectra in the form of a split-fringe containing first and secondbranches of the spectra which provides a detectable signal 18.Generally, such preselected spectra are those produced by scattering ofa major constituent of the gaseous material as, for example, therotational Raman spectra of oxygen or nitrogen in a sample of air. Asignal conditioning means 20 associated with the interferometric means16 measures the intensity of each branch. The intensity ratio of thebranches is measured by a detecting means 22, and used to calculate thetemperature of the gaseous material.

More specifically, as shown in FIG. 2, the interferometric means 16 hasinterference-producing means for providing a plurality of transmissionwindows regularly spaced in frequency. In addition, the interferometricmeans 16 has scanning means for variably controlling the frequency ofeach order. The frequency spacing between adjacent windows of theinterferometric means 16 is adjusted to depart from an odd integralsubmultiple, n, of the frequency difference between adjacent spectralcomponents of the periodic spectrum of a molecular species of thegaseous material, said odd integral submultiple being at least three, soas to produce the split-fringe. The scanning means is then adjusted sothat the transmission peaks for adjacent nth orders substantiallycoincide with the spectral lines of either branch of the components.When the interferometric means 16 is adjusted in the above manner, eachbranch of the split-fringe is derived from a plurality of periodicspectral lines and has an integrated intensity substantially equal totheir sum.

As previously noted, the light 14 which is subjected to analysis can bereceived from an external source. Generally, however, the light 14 isproduced by the apparatus 10. Hence, the apparatus 10 has light sourcemeans 26, such as a conventional argon ion laser, a frequency doubled,pulsed ruby laser or the like, for generating a highly monochromatic,coherent, collimated beam of radiation. The resolving power of theinterferometric means 16 is best utilized when the light source means 36is provided with means for projecting light having a line width andfrequency stability about equal to or less than the instrumental width,described hereinafter in greater detail, of the interferometric means16.

The use of a pulsed laser as the light source means 36 together with atime gated electronic detection system permits determination oftemperature and location of a sample of gaseous material remote from theapparatus 10. For example, by providing the apparatus 10 with (1) meansfor measuring the time interval required to send a laser pulse into thesample and receive a return signal caused by light scattered therein and(2) means for measuring the amplitude of the return signal, the distanceof the sample from the apparatus 10 as well as the temperature thereofis readily obtained. A pulsed laser adapted to determine temperature andlocation in the above manner preferably has means for projecting lighthaving a line width and frequency stability about equal to or less thanthe instrumental width of the interferometric means associatedtherewith. Such means typically includes a mode selecting etalondisposed in the laser cavity.

A projecting means associated with the light source means 36 introducesthe radiation, schematically represented by ray path 38, into gaseousmaterial in sample compartment 40 in one direction, which will beconsidered to be substantially vertical for convenience in referencingdirection, but may, of course, be in any direction desired. Ramanscattered radiation, hereinafter referred to as light 14, from thegaseous material in sample compartment 40 is collected, collimated andtransmitted to the interferometric means 16 by the light conditioningmeans 12, which may be a lens, or other suitable optical system. As longas the gaseous material contains molecules which are of the linear orsymmetric top variety, the light 14 will exhibit spectral componentsperiodic in frequency.

The signal conditioning means 20 has modulating means 42 for modulatingthe phase difference between interfering rays of light 14 transmitted bythe interferometric means 16 so as to compare the peak intensities ofthe branches of the split-fringe transmitted thereby. Signalconditioning means 20 also has synchronous (e.g., phase sensitive)detecting means for detecting the intensity ratio of the lattersplit-fringe, whereby the intensity ratio of the split-fringe can beindicated by the detecting means 22.

Several known interferometric means may be adapted to selectivelyseparate periodic spectra from the light 14. Preferably, theinterferometric means is a Fabry-Perot Interferometer (FPI) having amirror separation, d, adjusted to transmit all rotational lines of amolecular species of the gaseous material. The transmission function ofan FPI (I_(t)) can be given by the Airy formula: I_(t) = τ² [1+R²-2Rcosφ]⁻¹.I_(o) where τ + R + A = 1, I_(o) is the intensity of theincident light, and the phase difference φ is expressed as φ = 4πμωd forrays normal to the FPI mirrors. The symbols A, R and τ represent,respectively, the absorbance, reflectance and transmittance of the FPImirrors, μ is the refractive index of the medium between the FPImirrors, d is the FPI mirror separation, and ω is the frequency of theincident light expressed in wavenumbers. When cos φ is equal to unity,transmission maxima for I_(t) occur. Hence, φ =2πm, where m takes onintegral values and represents the order of interference. Thetransmission maxima for I_(t) are referred to in the specification andclaims as transmission windows. For a specific value of the mirrorseparation d, the FPI provides a plurality of transmission windowsregularly spaced in frequency. The frequency spacing, Δf, betweenadjacent windows (or spectral range) of the FPI is Δf = (2μd(⁻¹. Byvarying the mirror spacing, d, of the FPI, Δf can be adjusted to departfrom the frequency difference between adjacent spectral components of aspecific periodic spectrum by a preselected frequency difference as inthe order of about 160B² /ω_(o) n to 480B² /ω_(o) n and preferably about240B² /ω_(o) n to 320b² /ω_(o) n. If the rotational Raman spectrum of agas is used as the periodic spectrum, the FPI will behave as a combfilter having its transmission windows matched to the given periodicspectrum so as to transmit all of the Raman lines of the spectrum in theform of a split-fringe containing first and second branches of the linesand block the Rayleigh line when the mirror spacing is adjusted so that

    Δf = (4B/n ±240B.sup.2 /ω.sub.o n           (2)

where B is the rotational constant of a molecular species, orconstituent of the gas. The Rayleight line is blocked because it fallsbetween two FPI transmission windows. Moreover, in the Raman spectrum,the Stokes and anti-Stokes Raman lines are symmetrically positionedaround the Rayleigh line (atω=ω_(o)). The first two Raman lines (havingrotational quantum number, J, equal to zero) are shifted away from ω_(o)by a frequency of 6B, whereas the frequency separation of successiverotational lines is 4B. Continuous scanning of the FPI in the vicinityof ##EQU2## produces an interferogram having equally spaced verticallines of constant amplitude, which represent Rayleigh fringes at ω_(o),and a plurality of split-fringes, positioned between such verticallines, each of the split-fringes containing a first branch (composed ofthe Stokes rotational lines) and a second branch (composed of theanti-Stokes rotational lines). When Δf = 4B/n, the transmission peaksfor adjacent orders coincide with the adjacent rotational Raman lines soas to produce a 1:1 correspondence therewith, and the amplitude of theRaman fringe transmitted is a maximum. For values of Δf slightlydifferent from 4B/n, the transmission peaks for adjacent orders will notperfectly coincide with the Raman spectrum and the profile of the Ramanfringe transmitted by the FPI will split into such first and secondbranches.

In order to illustrate the manner in which the Raman fringe splits toform a Stokes branch and an anti-Stokes branch, the positions and peakintensities of the individual rotational Raman lines were plotted for amirror separation slightly larger than the mirror separationcorresponding to the center of the 4B/5 interference pattern fornitrogen. The result is shown in FIG. 4. This figure shows schematicallythe relative positions of the individual rotational Raman lines betweentwo 514.5 nm Raleigh fringes corresponding to the fringe numbers 12263and 12264. The peak rotational line intensities were calculated for agas temperature of 300 K and the nitrogen ground state rotationalconstants of B_(o) = 1.989506 cm⁻¹ and D_(o) = 5.48×10⁻⁶ cm⁻¹. For theStokes branch lines, the Raman frequency of the rotational line withquantum number J is

    ω.sub.s = ω.sub.o - (4B.sub.o - 6D.sub.o)(J + 3/2) + 8D.sub.o (J + 3/2).sup.3

and the corresponding peak line intensity is ##EQU3## where K is aproportionality constant, T is the absolute temperature and h, c, and kare Planck's constant, the speed of light and Boltzmann's constant,respectively. For the anti-Stokes branch, the corresponding Ramanfrequency and peak intensity are given by

    ω.sub.A = ω.sub.o + (4B.sub.o -6D.sub.o)(J + 3/2) - 8D.sub.o (J + 3/2).sup.3

and ##EQU4## The rotational lines shown in FIG. 4 are depicted as havingzero linewidth. In reality, each line has a finite width which is due tothe combined effects of the laser linewidth, Doppler broadening by thescattering process and instrumental broadening by the Fabry-Perotinterferometer. In order to determine the fringe profile, a computerprogram was written and tested which takes into account theaforementioned factors. For the purpose of calculation, it is asumedthat the laser line is gaussian shaped with a width of Δω_(o). Thislaser line was convolved with the Doppler broadened profiles for theRayleigh line and the individual Raman lines. A convolution was thenperformed with the instrumental transfer function of the interferometer.At specified fringe intervals, the contributions from the Rayleigh lineand all the individual Raman lines were summed to yield the fringeprofile at that particular mirror position.

The Fabry-Perot transfer function may be written as

    I(ω) = G.sub.A (ω) * G.sub.D (ω) * G.sub.S (ω) (5)

where

G_(A) (ω) = the Airy function

G_(D) (ω) = the mirror defect function, and

G_(S) (ω) = the scanning aperture function.

It can be shown mathematically that the Fourier transform of theconvolution of two or more functions is equal to the product of theFourier transforms of the individual functions. Therefore, the Fouriertransform of I(ω) is

    i(X) .tbd. F.T. [ I(ω)] = g.sub.A (X) . g.sub.D (X) . g.sub.S (X) (6)

where g_(A), g_(D) and g_(S) are the Fourier transform of G_(A), G_(D)and G_(S), respectively.

The Airy function, G_(A) (ω), can be written as ##EQU5## where γ= 2μd =the optical path between interfering rays. The Fourier transform ofG_(A) (ω) is ##EQU6## This transform is non-zero only for the discretevalues of X = Nγ. The mirror defect function, G_(D) (ω) can be expressedas

    G.sub.D (ω) = 1 for -[2γ F.sub.D ].sup.-1 ≦ω<[2γF.sub.D ].sup.-1 = o for all other ω(9)

where F_(D) = Defect finesse = 1/2 m for λ/m flatness figure. TheFourier transform of G_(D) (ω) is ##EQU7## Similarly, the scanningaperture function can be written as

    G.sub.S (ω= 1 for -[2γF.sub.S ].sup.-1 ≦ω< [2γF.sub.S ].sup.-1 =  0 for all other ω.     (11)

The scanning finesse, F_(S), is defined as ##EQU8## where θ_(p) is thepinhole angle in radius. The Fourier transform of G_(S) (ω) is ##EQU9##A broadened line profile, H (ω), is of the form

    H (ω) = H (ω.sub.R) exp [-(4 ln 2) (ω-ω.sub.R).sup.2 /(Δω.sub.R).sup.2 ](14)

where ω_(R) = Raman frequency (cm⁻¹)

H(ω_(R))= peak intensity of the individual Raman lines and

Δω_(R) = the Doppler line width. ##EQU10## where T = the absolutetemperature (K)

M = the molecular weight

R = the gas constant, and

φ = the scattering angle.

The Fourier transform of H (ω) is ##EQU11## A pressure broadened lineprofile, L(ω), may be represented by an Lorentzian function given by theequation:

    L(ω) = (Δω.sub.p /2)[(ω- ω.sub.R).sup.2 + (Δω.sub.p /2).sup.2 ].sup.-1                  (17)

where ω_(R) = the Raman frequency (cm ⁻¹)

Δω_(R) = the full width at half maximum of the pressure broadened line.

The fourier transform of L(ω) is

    l(X) = F.T. [L(ω)]= exp [-πΔω.sub.p X]. (18)

the convolution of all these functions is achieved by forming theproduct of the Fourier transforms and then taking the inverse Fouriertransform of the product. The calculation is simplified by the presenceof the δ-function in the Fourier transform of the Airy function, sinceit is necessary to compute the product only for discrete values of X =Nγwhere N = 1, 2, 3, etc.

The computer program begins by calculating the frequencies and peakintensities of the individual Raman lines. For a given optical pathdifference, γ, an array A(N) is calculated. The array A(N) is theproduct of the Fourier transforms of the FPI transfer function, agaussian lineshape for the exciting laser light, a gaussian lineshapefor the Doppler broadened scattered light and a Lorentzian lineshape forthe pressure broadened scattered light. The intensity of theinterferogram for a given γ is ##EQU12## where the index i runs over allspectral lines. The value of γ is incremented and the calculation isrepeated.

This computer program was used to calculate the fringe profile for thefringe interval shown in FIG. 4. The results of the calculation areshown in FIG. 5. The open circles represent the calculated profile forthis particular fringe interval and the five triangular points whichrepresent experimental data indicate that the agreement between theexperimental and calculated fringe profiles is quite good.

A computed fringe profile for a fringe interval corresponding to amirror separation slightly less than the mirror separation for thecenter of the 4B/5 interference pattern in nitrogen is shown in FIG. 6.Since the positions of the individual Raman lines within a given fringechanges as the mirror separation is changed, this ratio of the Stokesbranch intensity to the anti-Stokes branch intensity shown in FIG. 6differs from that shown in FIG. 5. In order to investigate the variationof this Raman intensity ratio as a function of fringe number (mirrorseparation), fringe profiles were calculated for three differenttemperatures for the fringe numbers 12250 to 12273 inclusive. The Ramanintensity ratios were calculated from the computer fringe profiles andthe results are plotted in FIG. 7 for nitrogen gas. The change in theRaman intensity ratio as a function of temperature was determined byarbitrarily selecting a fringe interval (12260 to 12261) and calculatingthe Raman fringe profiles for several different temperatures. FIG. 8 isa graphic representation showing the variation of the calculated splitfringe ratio over the temperature range 100K to 600K. Also shown in FIG.8 is the variation of the calculated split fringe ratio for the fringe12170 within the same temperature range.

A similar computer investigation was carried out for the 4B interferencepattern of nitrogen for selected fringe intervals on either side of the4B interference maxima (which occurs at the 514.5 nm order number of2443). These results are shown in FIG. 9. In practice, the Raman fringeintensity ratio is measured from a fringe number produced by anexperimentally determined mirror separation. For that particular fringe,a computer calculation is then performed using γ values coresponding topeak portions of the stokes and anti-stokes branches of the split-fringeto give the Raman fringe intensity ratio as a function of temperatureaccording to equation 19. Alternatively, the Raman fringe intensityratio is experimentally measured for several known gas temperatures inthe range of interest in order to calibrate the apparatus 10. FIGS. 8and 9 show that the Raman intensity ratio for the split fringe variesinversely with gas temperature, i.e., the ratio is greater at lowertemperatures than at higher temperatures.

The temperature of a preselected constituent of gaseous material canalso be determined by measuring the frequency difference betweenpreselected portions (preferably the peak portions) of first and secondbranches of the split fringe. Such a frequency difference is producedwhen the spectral range of the interferometric means is adjusted todepart from an odd integral submultiple, n, of the frequency differencebetween adjacent spectral components of the periodic spectrum of amolecular species of the gaseous material appointed for analysis, andthe scanning means is adjusted to cause transmission peaks for adjacentnth orders to substantially coincide with the spectral lines of eitherbranch of the components. The frequency difference between thepreselected portions of the first and second branches is measured by thesignal conditioning means, indicated and recorded by detecting means andcorrelated with the temperature of the gaseous material.

The variation in frequency difference as a function of temperature wasdetermined by means of the fringe profile computor program for nitrogengas. FIG. 10 is a graphic representation showing the computed variationof the frequency difference between the peak portions of thesplit-fringe over a temperature range of 100°-1000° K for the 514.53 nmorder number of 2410. FIG. 11 is a graphic representation showing thecomputed variation of the frequency difference between the peak portionsof the split-fringe over a temperature range of 100°-600° K for the514.53 nm order number of 12,260. For nitrogen gas at room temperaturethe frequency difference between the peak portions of the split-fringewas determined experimentally to be 0.556 cm⁻¹. From the computed datashown in FIG. 11, the temperature corresponding to the frequencydifference of 0.556 cm⁻¹ is equal to 297 K, or 24° C., which equaled,approximately, the ambient temperature.

For certain molecules, such as oxygen and carbon dioxide, spectralcomponents of the rotational Raman spectra having either even or oddrotational quantum numbers (J) will have zero intensity. Thisalternation in the intensity of the rotational Raman lines is producedby the effects of nuclear spin. For such molecules, adjacent rotationallines in the Stokes and anti-stokes branches are separated by afrequency substantially equal to 8B. Secondary interferograms areproduced for values of the interferometer spectral range equalsubstantially to 8B/n, where n is an odd integer. The secondaryinterferograms which are produced, consist of two Raman fringes betweenadjacent Rayleigh fringes: one Raman fringe being due to thesimultaneous transmission by the FPI of only Stokes Raman lines and theother Raman fringe being due to the simultaneous transmission by the FPIof only anti-Stokes Raman fringes. By measuring the intensity ratio ofthe peaks of these two fringes, gas temperatures may be deduced. Thistechnique will be useful for only molecules with either odd or even Jvalue lines missing.

As previously noted, a modulating means 42 is associated with theinterferometric means 16 for modulating the phase difference, φ, so asto compare the peak intensities of the branches of the split-fringetransmitted thereby. In order to obtain the maximum modulated signalfrom the split-fringe appointed for analysis, the modulating means isadjusted to modulate between the peak portion of each branch thereof.Generally speaking, the modulating range should be no greater than thefrequency spacing between adjacent orders.

The resultant signal 18 from the inerferometric means 16 is collectedand focused in the plane of pinhole stop 44 by a lens 46. Lens 46 isadjusted so that the center of the signal 18 is positioned on thepinhole 48. The intensity of the portion of signal 18 passing throughthe pinbhole 48 is detected by a photomultiplier 50. A phase sensitivedetection means 52, such as a lock-in amplifier, is adapted to receivethe signal from the photomultiplier 50 and detect the intensityvariation of the fringe appointed for analysis. The output of the phasesensitive detection means 52 is displayed by an indicating and recordingmeans 54, which can comprise an oscilloscope and a chart recorder.

In FIG. 3, the interferometric means 16 and the modulating means 42 areshown in greater detail. The interferometric means shown is aFabry-Perot Interferometer (FPI) which is scanned by varying the phasedifference, φ, between interfering beams of light in a conventional way.Scanning methods such as those wherein the pressure of gas between themirror of the FPI is altered so as to change the optical paththerebetween can also be used. Accordingly, interferometric means 16shown in FIG. 3 should be interpreted as illustrative and not in alimiting sense. Such means has cylindrical air bearings 56 and 58 whichnormally operate at about 30 psi and collectively support a hollow metalcylinder 60 approximately 35 cm. long and constructed of stainless steelor the like. The outer diameter of the cylinder 60 is centerless groundto about 4 cm. The inner diameter of the cylinder 60 is about 3.5 cm.Each of the air bearings 56 and 58 is about 8 cm. long and has outer andinner diameters of about 5 cm. and about 4 cm., respectively. Theseparation between centers of the air bearings is approximately 20 cm.One of the mirrors 62 of the interferometric means 16 is fixedly mountedon end 64 of cylinder 60 as by a suitable adhesive or the like. Theplane surface of the mirror 62 is substantially perpendicular to therotational axis of the cylinder. The other mirror 66 is fixedly mountedto the modulating means 42 as hereinafter described. Each of the airbearings 56 and 58 rests in precise V-blocks of a base plate (not shown)treated so as to dampen external vibrations. The light 14 to be analyzedenters the interferometric means 16 at end 68 of cylinder 60. A carriage70 caused to move horizontally by means of a precision screw 72 andhaving a coupling arm 82 fixedly secured thereto by mechanical fasteningmeans, such as screws 88, and to cylinder 60 as described hereinafterprovides the cylinder 60 with the linear motion needed to scan theinterferometric means 16. Precision screw 72 is coupled to a digitalstepping motor 74 through gear assembly 76. The scan rate of theinterferometer is controlled either by changing the gear ratio ofassembly 76, as by means of magnetic clutches or the like, or by varyingthe pulse rate input to the digital stepping motor 74. With apparatus ofthe type described the scan rate can be varied over a range as great as10⁶ to 1 or more.

In order to transmit precisely the linear motion to cylinder 60, acollar 78 having glass plate 80 adhesively secured thereto, is fixedlyattached to the cylinder 60. The coupling arm 82 has a ball 86 comprisedof stainless steel, or the like, associated with an end 84 thereof. Apermanent magnet 90 is attached to end 84 of coupling arm 82 near theball 86. Due to the magnetic attraction between the collar 78 and themagnet 90, the ball is held in contact with the glass plate 80. A lowfriction contact point is thereby provided. The contact force producedat such contact point by linear movement of the carriage 70 can beadjusted either by varying the separation between the magnet 90 and thecollar 78, or by decreasing the strength of the magnet 90.

A sectional view of one form of modulating means 42 is shown in FIG. 3.Other forms of the modulating means 42 can also be used. Preferably, themodulating means 42 has a hollow cylindrical body 92 of piezoelectricceramics. The inner and outer wall 94 and 96 of the cylindrical body 92are coated with an electrically conductive material such as silver orthe like.

Insulating members 98 and 100 comprised of an insulating material suchas ceramic or the like are secured to the cylindrical body 92, at ends102 and 104, respectively, by a suitable adhesive such as an epoxyresin. Mirror 66 is fixedly attached to insulating member 98 by anadhesive of the type used to secure mirror 62 to the end 64 of cylinder60. In order that mirror 66 be maintained in parallel with mirror 62,the insulating member 100 is adhesively secured to face 106 of holdingmember 108. The outer face 110 of the holding member 108 has connectedthereto a plurality of differential screw micrometers 112, which can beadjusted in the conventional way to provide for precise angularalignment of the mirror 66. Electrodes 114 and 116 are attached to theinner wall 94 and the outer wall 96, respectively. Voltage having a waveform such as a sine wave or a square wave impressed thereon is appliedfrom a high voltage low current power supply 101 to the electrodes 114and 116. Upon application of the voltage the cylindrical body 92 iscaused to modulate in a linear direction whereby the intensity of signal18 is varied. When the voltage applied from power supply 101 toelectrodes 114 and 116 has the form of a square wave, the voltage limitsof the wave form can be adjusted so that the intensity of thesplit-fringe to be analyzed from signal 18 alternates between themaximum values of the branches. A detection means is provided fordetermining the photon cokunt at the peak of each branch of thesplit-fringe for each half-cycle of the square wave to produce first andsecond signal counts, accumulating the signal counts for a preselectedperiod of time over a preselected number of cycles of the square waveand dividing the first signal count by the second signal count toproduce a signal count ratio, the preselected time period andpreselected number of cycles varying inversely with the branchintensities of said split-fringe. As a result, the accuracy of thedetecting means and hence the sensitivity of the apparatus 10 isincreased by a factor in the order of about 100 or more.

The apparatus 10 which has been disclosed herein can, of course, bemodified in numerous ways without departing from the scope of theinvention. For example, interferometric means 16 can be a fixed etalontuned by controlling the temperature thereof. One type of fixed etalonwhich is suitable is comprised of optically transparent material, suchas fused silica, having opposed surfaces which are polished, flat,parallel and coated with silver, dielectric material or the like forhigh reflectivity at a preselected frequency region. The thickness ofthe etalon can be chosen so that the free spectral range of the etalondeparts from an odd integral submultiple, n, of the difference betweenadjacent spectral components of the periodic spectrum of a molecularspecies of the gaseous material by the preselected frequency spacing,thereby producing a split-fringe containing first and second branches ofthe components. Fine tuning of the solid etalon is affected by providingmeans for controlling the temperature, and hence the optical pathlength, thereof so as to cause the transmission peaks for adjacent nthorders to substantially coincide with the spectral lines of eitherbranch of the components. As previously noted, the light 14 to beanalyzed need not be Raman scattered light solely but can be any lightfrom the visible, infrared or ultraviolet frequency regions which hasspectral components periodic in frequency. The increased sensitivity ofthe apparatus makes it especially suited for temperature measurement atdistant locations of gaseous mateial. Hence, the gaseous material neednot be located within a sample compartment, but may instead be locatedat points distant from the apparatus 10, as in the order of up to about10 miles distance therefrom. Other similar modifications can be madewhich fall within the scope of the present invention. It is,accordingly, intended that all matter contained in the above descriptionand shown in the accompanying drawings be interpreted as illustrativeand not in a limiting sense.

In operation of the preferred apparatus, light 14 produced by scatteringin gaseous material and having spectral components periodic in frequencyis collected, collimated and trasmitted by light conditioning means 12to interferometric means 16. The interferometric means 16 receives thelight 14, selectively separates therefrom preselected periodic spectra,and transmits the spectra in the form of a split-fringe containing firstand second branches of the components and providing a detectable signalcorrelated with the temperature of the gaseous material. A modulatingmeans 42 operates to modulate the phase difference of the primaryinterferometric means so as to compare the peak intensities of eachbranch of the split-fringe. The intensity ratio (or, alternatively, thefrequency difference between preselected portions) of the branches ofthe split-fringe is detected by a phase sensitive detection means 52.The resultant signal from the phase sensitive detection means 52 isdisplayed by the indicating and recording means 54.

Having thus described the invention in rather full detail, it will beunderstood that these details need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the present invention asdefined by the subjoined claims.

I claim:
 1. Apparatus for determining the temperature of a gaseousmaterial by analyzing light having spectral components periodic infrequency, comprising:a. light conditioning means for collecting,collimating and transmitting said light; b. interferometric meansadapted to receive said light for selectively separating periodicspectra therefrom and transmitting said spectra in the form of adetectable signal, said interferometric means havinginterference-producing means for providing a plurality of transmissionwindows regularly spaced in frequency, the frequency spacing betweenadjacent windows being adjusted to depart from an odd integralsubmultiple, n, of the frequency difference between adjacent spectralcomponents of the periodic spectrum of said gaseous material, said oddintegral submultiple being at least three so as to produce asplit-fringe containing first and second branches of the components, andscanning means for causing the transmission peaks for adjacent nthorders to substantially coincide with the spectral lines of eitherbranch of the components, whereby each branch of said split-fringe isderived from a plurality of periodic spectral lines and has anintegrated intensity substantially equal to their sum; c. signalconditioning means for measuring the intensity of each of said branches;and d. detecting means for indicating and recording the intensity ratioof the branches, said intensity ratio correlating with the temperatureof said gaseous material.
 2. Apparatus as recited in claim 1, whereinsaid signal conditioning means includes modulating means for modulatingthe phase difference between interfering rays of said light so as tocompare the peak intensities of said branches of said split-fringe, saidmodulating range being no greater than the frequency spacing betweenadjacent nth orders, and synchronous detection means for detecting theintensity ratio of said split-fringe.
 3. Apparatus as recited in claim2, wherein said modulating means has a modulating range substantiallyequal to the frequency difference between peak intensity portions ofsaid branches of said split-fringe.
 4. Apparatus as recited in claim 2,wherein said modulating means is a piezoelectric cylinder and saidsynchronous detection means is a phase-sensitive detection system. 5.Apparatus as recited in claim 4, wherein said interferometric means is asolid etalon having temperature control means associated therewith foradjusting the optical path length thereof.
 6. Apparatus as recited inclaim 4, including means for applying to said cylinder a voltage havinga square wave form, the limits of said voltage being adjusted so thatthe intensity of said split-fringe alternates between the maximum valuesof said branches, means for determining for each half-cycle of saidvoltage the photon count at peak intensity of each branch of saidsplit-fringe to produce first and second signal counts, means foraccumulating the signal counts for a preselected period of time over apreselected number of cycles of said square wave, and means for dividingthe first signal count by the second signal count to produce a signalcount ratio.
 7. Apparatus as recited in claim 6, including means forvarying the preselected period of time, and the preselected number ofcycles inversely with the branch intensities of said split-fringe. 8.Apparatus as recited in claim 4, wherein said phase sensitive detectionsystem is a lock-in amplifier.
 9. Apparatus as recited in claim 4,wherein said synchronous detection means is a photon counting system.10. Apparatus as recited in claim 1, including light source means forgenerating monochromatic light, and projecting means for directing saidmonochromatic light through said gaseous material to produce saidscattered light having spectral components periodic in frequency. 11.Apparatus as recited in claim 10, wherein said light source means isprovided with means for projecting light having a line width andfrequency stability about equal to or less than the instrumental widthof said interferometric means.
 12. Apparatus as recited in claim 10,wherein said light source means is a pulsed laser.
 13. Apparatus asrecited in claim 12, wherein said laser is associated with a time gatedelectronic detection system having (1) means for measuring the timeinterval required to send a pulse from said laser into said gaseousmaterial and receive a return signal caused by light scattered thereinand (2) means for measuring the amplitude of said return signal. 14.Apparatus as recited in claim 1, wherein said interferometric means is aFabry-Perot interferometer.
 15. A method of determining the temperatureof a gaseous material by analyzing light having spectral componentsperiodic in frequency, comprising the steps of:a. collecting,collimating and transmitting said light in the form of a ray path; b.interferometrically separating periodic spectra from said light bydirecting said light through a plurality of transmission windowsregularly spaced in frequency, the frequency spacing between adjacentwindows being adjusted to depart from an odd integral submultiple, n, ofthe frequency difference between adjacent spectrum of said gaseousmaterial said odd integral submultiple being at least three so as toproduce a split-fringe containing first and second branches of thecomponents, and scanning said ray path to cause the transmission peaksfor adjacent nth orders to substantially coincide with the spectrallines of either branch of the components; c. transmitting a detectablesignal composed of said split-fringe, each branch of said split-fringebeing derived from a plurality of periodic spectral lines and having anintegrated intensity substantially equal to their sum; d. measuring theintensity of each of said branches; and e. detecting and recording theintensity ratio of the branches, said intensity ratio correlating withthe temperature of said gaseous material.
 16. A method as recited inclaim 14, wherein said gaseous material comprises a mixture of gases andsaid preselected spectra are the spectra of a major constituent thereof.17. A method as recited in claim 15, wherein said gaseous material isair.